**1. NAME AND TITLE**

KIM: A Two-Dimensional Monte Carlo Code System for Linear Neutron Transport Calculations.

**2. CONTRIBUTOR**

CNEN, Dipartimento Ricerca Tecnologica di Base ed Avanzata, Bologna, Italy, through the OECD NEA Data Bank, Gif-sur-Yvette, France.

**3. CODING LANGUAGE AND COMPUTER**

Fortran IV, Assembler language; IBM 360/370/3033.

**4. NATURE OF PROBLEM SOLVED**

KIM (k-infinite-Monte Carlo) solves the steady-state linear neutron transport equation for a fixed source problem or, by successive fixed-source runs, for the eigenvalue problem, in a two-dimensional infinite thermal reactor lattice using the Monte Carlo method. In addition to the combinatorial description of domains, the program allows complex configurations to be represented by a discrete set of points whereby the calculation speed is greatly improved. Configurations are described as the result of overlays of elementary figures over a basic domain.

**5. METHOD OF SOLUTION**

KIM uses the Monte Carlo method. It is organized in three sections for fast, epithermal, and thermal simulation. Each section implements a particular model; both numerical techniques and cross-section representation vary with the energy section.

The tracing of the neutron history takes into account both the slowing down and the thermalization phenomena. During the slowing down (energy above 1 eV), nuclei are considered as stationary, with the exception of some resonance nuclei with spacing between resonances much greater than the resonance widths (Doppler broadening included). The Doppler broadening of s-wave resonances of these nuclei is taken into account during the simulation by computing cross sections at the current energy of the neutron and at the temperature of the nucleus hit. During thermalization (energy below 1 eV) the thermal motion of some nuclides is also considered. In the present data library, thermalization kernels at several temperatures are given for water, heavy water, and oxygen.

The main quantities computed in each of the three sections mentioned above are: (1) fluxes and cross sections averaged over the region, grouping of regions, the cell; (2) absorption and fission rates per nuclide, averaged over the region; and (3) flux, absorption, and production distributions vs energy averaged over the region.

**6. RESTRICTIONS OR LIMITATIONS**

None noted.

**7. TYPICAL RUNNING TIME**

The time needed on the IBM 370/168 to obtain the infinite multiplication factor with a precision of about 0.3%, for a typical 8 × 8 rod element of a BWR, for 40,000 histories, is about 40 minutes. This time refers to geometry treated in the discrete mode; the continuous mode requires almost double the time.

**8. COMPUTER HARDWARE REQUIREMENTS**

The code is operable on the IBM 360/370/3033. The computer memory requirement is problem-dependent, through dynamic core allocation at running time for the most critically-sized arrays (as, for example, thermalization kernels and the map of the discretized domain). Most cases will run in about 1000 K bytes.

**9. COMPUTER SOFTWARE REQUIREMENTS**

A Fortran IV or Assembler compiler is required.

**10. REFERENCE**

E. Cupini, A. De Matteis, and R. Simonini, *"*KIM A Two-Dimensional Monte Carlo Program
for Thermal Reactors," CNEN-RT/FIMA(80)2.

**11. CONTENTS OF CODE PACKAGE**

Included are the referenced document and one (1.2MB) DOS diskette which contains the source code and sample problem input and output.

**12. DATE OF ABSTRACT**

February 1982; revised February 1983.

**KEYWORDS: ** TWO-DIMENSIONS; MONTE CARLO; NEUTRON; CONBINATORIAL
GEOMETRY; THERMALIZATION; LWR